1. Field of the Invention
This invention relates to the control system for a helicopter rotor, more particularly it relates to the position of the upper controls that drives the rotating ring and holds the stationary ring.
2. Description of the Prior Art
Conventional helicopter rotor control systems drive the swashplate rotating ring through scissor assemblies, which include spherical rod end bearings whose average service life is short because their failure rate is higher than the average for components of the system. A large number of maintenance problems occur in the swashplate assembly due to wear, binding, looseness and excessive end play of duplex bearings present in the assembly. These problems with swashplate bearings are generally attributed to peak loads applied to the swashplate by the boost actuator input by the lower controls.
In conventional rotor assemblies for helicopters the swashplate is centered on the axis of the rotor shaft by a ball slider contacting a slider guide shaft. Ball slider guide assemblies are costly and prone to wear. Looseness in the rotor control system resulting from wear degrades flying quality, and the removal and replacement of the ball slider assembly is a major overhaul task. For military helicopters the vulnerability of ball slider assemblies to jamming by, for example, a 12.7 mm round is a serious concern.
These difficulties can be overcome if the rigid metal components of the rotor control system and the spherical rod end bearings and ball bearing required to permit rotational freedom among the components of the system were replaced with flexible members able to deflect under load sufficiently to permit the required rotational freedom. Modern composite materials such as fiber reinforced, polymer resin matrices have properties particularly suitable for this purpose. When such materials are used to form large thin components the requisite flexibility is inherent and their size permits the control forces and rotor load to be carried by the components at relative low stress levels compared to metal components.
Various attempts to incorporate flexible diaphragms in helicopter control systems have been made. U.S. Pat. No. 4,326,834 describes the use of a pair of annular flexible diaphragms located in the rotor hub and connecting the rotor shaft to the radially outer periphery of the rotor hub. These diaphragms are pretensioned and operate to resist axial displacement of the hub relative to the rotor shaft and to resist angular displacement of the plane of rotation of the hub relative to a place normal to the rotor shaft axis. A sleeve rotates with the rotor shaft and has a plate connecting the sleeve to the pitch links for canting the rotor hub as the sleeve is moved on the rotor shaft. Cyclic pitch is transmitted as the sleeve structure tilts relative to the sleeve but the swashplate does not move vertically for collective input and tilt for cyclic input. If cyclic control is held fixed and collective control is moved the control system does not transmit collective pitch control from the sleeve to the sleeve structure because cables directly attach the sleeve structure to the cyclic control stick. It is preferable to transmit collective and cyclic control from the actuators to the blades without feedback or lockout.
Flexible diaphragms used to connect and partially support helicopter rotor blades on the rotor shaft are described in U.S. Pat. Nos. 4,566,856; 4,569,629; and 4,580,945.